Composite filter media find wide use in the purification of fuel streams. In these fuel stream applications the filters allow significant fluid flow through their structures while offering benefits of solid particulate removal. The composite media employed often comprise several discrete layers and these layers can be bonded together in a laminate at discrete points of contact or held in place by cooperative seals at their edges. One drawback common to current media is a high resistance to flow of fuel fluids through their structures resulting in high operating differential pressures. Furthermore these same composite structures exhibit a low resistance to the penetration of water when immersed in a fuel stream. Here the water in the fuel stream may take the form of particles, chunks, slugs, streams, or film forming layers that are a separate discrete phase from the fuel.
Composite filter media known in the art include a wide variety of porous and composite materials. The porous composite media most commonly employed in practice are comprised of fibrous polymer non-wovens, cellulose or paper non-wovens, including those containing microfiberglass, and textiles. Many of the aforementioned media also include hydrophobic coatings. Examples also exist of fibrous and non-fibrous microfilter membranes including fully and partially fluorinated polymers and expanded polytetrafluoroethylene (ePTFE), for example those described in US 2008/0105629 A1.
In general, these composite filter media of the prior art provide only low resistance to water penetration in fuel streams. In a variety of applications it is desirable to have media that exhibits low resistance to fuel flow with a high resistance to water penetration. This is particularly desirable in application of these composites in fuel filters used for middle distillate fuels in the aviation and automotive industries. For example, in a variety of fuel purification endeavors it is desirable to have a media that exhibits a high resistance to water penetration so as to allow consistent water removal over a range of flow rates and differential pressures experienced by a fuel filter.
The applicants have discovered that the resistance of a media to water penetration when wet with fuel can be quantified in terms of the pressure required to drive water to penetrate through the media in a laboratory test of Fuel Wet Water Penetration Pressure, referred to hence forth as the FWWPP test. Furthermore, it has been discovered that the FWWPP value is influenced by both the quality of the fuel in terms of its interfacial tension against water (IFT) and the structure of the composite filter media. A useful metric for water penetration resistance is thus given by the ratio of FWWPP/IFT.
While a resistance to water penetration is desired, what is truly missing from current known composite media is a combination of high resistance to water penetration with low resistance to flow in a composite media. The applicants have also found that the airflow of the base media is inversely related to the resistance to fuel flow of many composite structures. Therefore a useful figure of merit can be defined as the ratio of water penetration resistance FWWPP/I FT to resistance to flow as represented here by the airflow resistance (1/ATEQ Airflow Value). Henceforth, herein this ratio of (FWWPP/IFT)/(1/ATEQ Airflow Value) is referred to as the Ratio of Resistances for Water Penetration to Flow. In addition to high water penetration resistance, and a high Ratio of Resistances, in some applications, it is desirable for a fuel filter media to prevent penetration of a bulk water slug through the filter media to pressures exceeding the maximum driving pressure of the supply pump, particularly when the water takes the form of a water slug which excludes fuel from the upstream plumbing and filter media surfaces. This is particularly important for filters immersed in fuel streams that end in the fuel tanks of jet airplanes and trucks. The passage of water slugs through said filter media can result in catastrophic icing of aircraft fuel lines or major damage to the fuel injectors of modern diesel engines.
The prior art includes known examples of technologies capable of limiting or reducing water passage in the presence of water slugs. These technologies typically comprise a super absorbent polymer (SAP) and combinations of fibrous nonwovens and woven materials. See for example U.S. Pat. No. 4,959,141, U.S. Pat. No. 6,997,327 and U.S. Pat. No. 7,998,860 and references therein. On contact with water the SAP polymer takes up the water swelling shut the filter media and preventing further fuel or water flow. This media can provide significant resistance to water penetration (FWWPP/IFT) e.g. >5000 PSI/(lb/ft), However this technology typically reduces or shuts off fuel flow, further increasing the resistance of said filter media to flow and so often exhibits a poor resistance ratio on exposure to even small quantities of water.
In addition, the resistance to water penetration from these materials is not typically robust because swelling of the SAP polymer is a kinetic phenomenon. Therefore, water rejection depends closely on the flow rate, operating conditions, and quantity of water introduced. Furthermore, the SAP technology lacks robustness to repeated pressure spikes and certain fuel system icing inhibitors such diethylene glycol monomethyl ether and other polar compounds such as biofuels which can result in extrusion, migration, or leaching of super absorbent polymer into the fuel stream with deleterious consequences. For example jet engine flame out and surface residue have been linked to the migration of this super absorbent polymer.
Another technology known in the art is the use of a fibrous non-woven comprised of cellulose, fiber glass, or polymeric fibers or a composite thereof. Such non-wovens exhibit varying degrees of resistance to flow and typically provide negligible resistance, <70 PSI/(lb/ft), to water penetration when wet with fuel. Furthermore, such materials are often coated with hydrophobic or water repellant polymers to increase their respective resistance to water penetration. Despite application of these treatments these materials typically exhibit low resistance, <1500 PSI/(lb/ft) (FWWPP/IFT), to water penetration when wet with fuel and have a low Ratio of Resistances<10400 (Water Penetration/Flow) [((PSI/(lb/ft))/(hr/liter)].
Examples also exist of fibrous microfilter membranes of expanded polytetrafluoroethylene (ePTFE), for example those described in US2008/0105629 A1. These prior art materials can provide some resistance to water penetration in fuel streams ranging from 30 to 4000 PSI/(lb/ft). However these materials presently exhibit significant resistance to flow and thus have relatively low Ratios of Resistance ratios under 10400 (Water Penetration/Flow) [((PSI/(lb/ft))/(hr/liter)] when they exhibit significant water penetration resistance.
Thus, there is a need for a filter media that exhibits a high resistance to water penetration >10500 PSI/(lb/ft) with a high Resistance Ratio in order to allow for effective water separation under a variety of operating conditions covering a range of flows and differential pressures. In addition there is a need for a filter media that prevents penetration of water slugs to the down stream when wet with fuel and faced with a high impinging pressure. Furthermore, there is a long-felt need for a filter media capable of preventing the penetration of water slugs at high impingement pressures without the possibility of deleterious leaching, extrusion, or migration of super absorbent polymer.